Fep pellet

ABSTRACT

An FEP pellet having a volatile content of 0.2% by weight or less. The FEP pellet satisfies the following requirements (i) and (ii) when used to form a insulating material coating a core wire by extrusion coating at a coating speed of 2,800 ft/min.: (i) an adhesive strength between the insulating material and said core wire of 0.8 kg or more; and (ii) an average number of cone-breaks in the insulating material of one or less per 50,000 feet of the coated core wire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an FEP pellet and more specifically, anFEP pellet having improved moldability which can suitably be used incoating extrusion, particularly in high-speed molding, for molding aninsulated cable such as a wire and a cable.

2. Description of the Related Art

Tetrafluoroethylene (TFE)/hexafluoropropylene (HFP) copolymer hassuperior heat resistance, chemical resistance, extrusion moldability andthe like, and in addition, has a superior electric insulating propertyand high-frequency property with a low dielectric tangent. Therefore, itis used for insulating cable such as a cable and a wire, and suchinsulated cable is suitably used as a communication cable. Thecommunication cable includes a data transmission cable such as a LANcable.

TFE/HFP copolymer also has low flammability and low smoking properties.Thus, insulated cable made from such a copolymer can be used as a plenumcable, which is laid, for example, on the back of a ceiling of abuilding (plenum area) and strictly regulated for preventing the spreadof fire.

The insulated cable comprises a core wire such as a cable and aninsulating material formed from a resin such as a TFE/HFP copolymercoating the core wire. In general, the insulated cable is manufacturedby extrusion coating in which molten resin is extruded in the shape of atube, drawn down by inserting a core wire through the center portion ofthe resin tube in its axial direction, and the core wire coated with theresin is then taken up.

The term “draw-down” as used herein means extending a molten resinextruded from a die having an opening of relatively large sectional areato its final intended dimensions. The draw-down is characterized by adraw-down ratio (DDR), which is the ratio of the sectional area of theopening of the die to the sectional area of the insulated material ofthe final product. In general, the draw-down ratio is suitably from 50to 150. Because draw-down extends the resin in the above-describedmanner, elongation melt breakage (cone-breaks) which are discontinuousportions generated in the insulating material, tend to occur.

The term “insulated” cable as used herein means a cable or wire coatedwith an insulating material.

In recent years, an increase in molding speed has been desired toenhance productivity and to reduce cost, and there is a demand toincrease the speed at which the insulated core wire is taken up tothereby increase the coating rate. When the coating rate is increased,the insulating material thus obtained generally tends to suffer fromcone-breaks as a result of draw-down even if the draw-down ratio is thesame as used at lower coating rates. Moreover, adhesion to the core wireis lowered.

A TFE/HFP copolymer which can withstand an increased coating rate is indemand. Although the TFE/HFP copolymer is manufactured, e.g., bywater-soluble emulsion polymerization or the like, the polymer thusobtained has a functional group such as a carboxyl group originatingmainly from a reaction initiating agent at the dyads or ends of the mainchain thereof. Thus, a resulting problem is that the polymer generatesfoaming in a high-temperature atmosphere such as during melt processingto thereby cause cone-breaks.

In order to avoid this problem, conventional TFE/HFP copolymers aregenerally subjected to end stabilizing treatment. The end stabilizingtreatment includes changing an unstable group at the end to a stablegroup such as a difluoromethyl group or eliminating an unstable group byapplying high temperature and/or high shearing force afterpolymerization as disclosed, for example, in U.S. Pat. No. 4,626,587,Japanese Kokai Publication Hei-10-80917 and the like.

The TFE/HFP copolymer to which such end stabilizing treatment has beencompletely applied has a low volatile content. However, if it is usedfor the coating extrusion, adhesion with the core wire is inferior, andin particular, a copolymer having a completely fluorinated and groupcauses severe shrink-back. Such a molding method is disclosed in WO00/44797 and the like which comprises using a TFE/HFP copolymer having amelt flow rate (MFR) of 24 (g/10 min.) and a coating speed set at lessthan 2,000 feet/min. However, since the coating speed has furtherincreased in recent years, the problem of shrink-back may have worsened.

Because a conventional TFE/HFP copolymer is subjected to hightemperature and high shearing force at the end stabilizing treatment,the molecular weight distribution is generally narrow. If such aconventional TFE/HFP copolymer is used for the coating extrusion and ifan attempt is made to increase the coating speed, stable molding can becarried out at a low to medium speed, however, when the speed exceeds acertain extent, cone-breaks can suddenly occur.

For increasing the critical speed below which cone-breaks do not occur,for example, Japanese Kokoku Publication Hei-2-7963 discloses a processfor increasing melt tension by extending the molecular weightdistribution. In the embodiment of this process, however, one having lowcoating speed, and a TFE/HFP copolymer having an MRF of 14 (g/10 min.)or less is disclosed.

Die swell is considered to be an index for the molecular weightdistribution. In order to prevent cone-breaks by increasing thedraw-down rate during coating extrusion, the die swell preferably has alarge value. In a conventional TFE/HFP copolymer, however, because thedie swell ordinarily decreases remarkably during end stabilizingtreatment or the pelletizing step to cause melt fracture, high-speedcoating extrusion is difficult.

Although a process of coating extrusion of a cable with a TFE/HFPcopolymer having a certain die swell is disclosed in WO 01/36504, thisprocess specifies, as the TFE/HFP copolymer, a powder obtained bypolymerization.

In order to increase the speed of the coating extrusion, it is generallypreferred to reduce the melt viscosity of the resin. On the other hand,resistance to stress cracking of the resin decreases because of thelowered melt viscosity.

U.S. Pat. No. 4,029,868 and Japanese Kokoku Publication Sho-63-2281propose to carry out copolymerization using perfluoro(alkyl vinyl ether)(PFAVE) as a third monomer component of a TFE/HFP copolymer in order toprovide resistance to stress cracking.

U.S. Pat. Nos. 5,677,404 and 5,703,185 disclose a TFE/HFP copolymerobtained by carrying out copolymerization using PFAVE as a thirdcomponent. These patent publications disclose that a PEVE-based TFE/HFPcopolymer comprising perfluoro(ethyl vinyl ether) (PEVE) is superior inMIT bending life as compared to a PPVE-based TFE/HFP copolymercomprising perfluoro(propyl vinyl ether) (PPVE) as PFAVE, hence it ispossible to increase the coating speed.

However, no mention is made to the adhesive strength of the insulatingmaterial to the core wire, or to cone-breaks.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems of the prior art, an object ofthe present invention is to provide an FEP pellet for extrusion coatingof cable with no occurrence of elongation melt breakage such ascone-breaks as well as having excellent adhesive strength between aninsulating material and a core wire at high coating speeds such as 2,800ft/min.

The FEP pellet of the present has a volatile content of 0.2% by weightor less, and satisfies the following requirements (i) and (ii) when usedto form an insulating material coating a core wire by extrusion coatingat a coating speed of 2,800 ft/min.:

(i) an adhesive strength between the insulating material and the corewire of 0.8 kg or more; and

(ii) an average number of cone-breaks in the insulating material of oneor less per 50,000 ft of the coated core wire.

The invention is described in further detail below.

BRIEF DESCRIPTION OF THE DRAWING

The FIG. 1 illustrates the measuring device (metal plate having acolumnar hole) used to evaluate adhesive strength of the insulatingmaterial with the core wire.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, FEP pellet means a pellet comprising a copolymerobtained by copolymerizing monomer components containingtetrafluoroethylene (TFE) and hexafluoropropylene (HFP). In the presentspecification, the above-mentioned copolymer is referred to as “FEP”.

The above-mentioned FEP may be a binary copolymer using solely TFE andHFP as monomer components. As used herein, the above-mentioned binarycopolymer is referred to as “unmodified FEP”.

The above-mentioned FEP may also be a ternary copolymer containing unitsderived from TFE, HFP and a small amount of a perfluoro vinyl ether asmonomer components. As used herein such a ternary copolymer is referredto as “modified FEP”. One, two or more species of the above-mentionedperfluoro vinyl ether may be used.

The above-mentioned perfluoro vinyl ether is not particularly limitedand, for example, includes an unsaturated perfluoro compound representedby the following general formula:

CF₂=CF—OR_(f)

where R_(f) is a perfluoroaliphatic hydrocarbon group. As used herein, aperfluoroaliphatic hydrocarbon group means an aliphatic hydrocarbongroup in which all of hydrogen atoms linked to the carbon atoms arereplaced with fluorine atoms. The above-mentioned perfluoroaliphatichydrocarbon group may have one or more ether oxygens.

An example of the above-mentioned perfluoro vinyl ether includesperfluoro(alkyl vinyl ether) (PFAVE). PFAVE is a compound represented bythe following general formula:

CF₂═CFO(CF₂)_(n)CF₃

where n is an integer of 0 to 3. One, two or more species of PFAVE maybe used. As PFAVE, perfluoro(ethyl vinyl ether) (PEVE) andperfluoro(propyl vinyl ether) (PPVE) are preferred, and PPVE is morepreferred since it imparts superior MIT bending life as described below,and can readily increase the speed of the coating extrusion.

As the above FEP, modified FEP comprising PFAVE as the perfluoro vinylether is preferable since it can increase the resistance to stresscracking. As used herein, such modified FEP is referred to as“PFAVE-modified TFE/HFP copolymer”.

The weight ratio of the contents of TFE, HFP and optionally PFAVE asmonomer components of the above-mentioned FEP is not particularlylimited as long as the FEP pellet of the present invention has thebelow-described characteristics, however, the weight ratio ofTFE:HFP:PFAVE is preferably 70 to 90:10 to 20:0 to 10. If the content ofPFAVE is within the above range, sufficient draw-down is possible, andit is also possible to maintain high resistance to stress cracking. Morepreferably, the ratio is 75 to 95:10 to 20:0 to 5.

The percentage of the TFE, HFP and PFAVE contents as monomer componentsof the above-mentioned FEP is also called a monomer composition, and isa value obtained by NMR analysis. The monomer composition can also bemeasured by infrared absorption spectrum analysis as described inJapanese Kokoku Publication Sho-63-2281.

The FEP pellet according to the present invention may contain othercomponents as needed. The above-mentioned additional components are notparticularly limited and include, for example, various kinds of fillers,stabilizers, lubricants and other conventional additives.

The FEP pellet of the present invention is suitably used in coatingextrusion for insulating a core wire. The above-mentioned FEP pellet ismelted by heating within an extruder for coating a core wire andextruded from a die, and then drawn down by coating the core wire tothereby form an insulated cable. The above-mentioned insulated cablecomprises a core wire coated with the extruded FEP material.

As used herein, the speed at which the core wire coated by the abovecoating extrusion is reeled up is referred to as “covering speed”. Thecoating speed is expressed by the length of insulated core wire lengthreeled up over a period of one minute (expressed in feet “ft”).

The FEP pellet according to the present invention satisfies thefollowing requirements (i) and (ii) when the above coating extrusion iscarried out at a coating speed of 2,800 ft/min:

(i) the adhesive strength between the above-mentioned insulatingmaterial and the core wire is 0.8 kg or more.

An adhesive strength between the insulating material and the core wireof, for example, 3 kg or less will be sufficient as long as it is 0.8 kgor more. It is preferably 0.8 to 2 kg, more preferably 1 to 2 kg.

The adhesive strength between the insulating material and the core wiredecreases as the coating speed increases. This relates to deteriorationof the resin. In general, when the coating speed increases under aconstant temperature profile, the temperature of the resin increases,which shortens the retention time of the resin in the extruder. Althoughit may be considered that the increase of the resin temperature maydegrade the resin, studies conducted by the present inventors revealthat the extent of degradation of the resin is more severely affected bythe retention time of the resin rather than an increase in the resintemperature. However, the FEP pellet of the present invention isadvantageous in that the adhesive strength with the core wire of theextruded resin is sufficiently high even if the coating speed isincreased and the retention time of the resin is shortened as mentionedabove.

The reason why the FEP pellet of the present invention providesexcellent adhesive strength is not clear. However, it is considered thatthe above-mentioned FEP has a functional group which can be changed tocontribute to increased adhesion with the core wire when extruded athigh temperature. As used herein, such functional group is referred toas an “adhesion factor”, and if the adhesion factor is at the end of thepolymer, such end is referred to as an “adhesion terminus”.

The reason why the above adhesion terminus adheres to the core wire isnot clear. However, it is considered that the increased adhesion is dueto chemical reaction and/or high affinity with the core wire.

The above-mentioned adhesion terminus is not particularly limited aslong as it contributes to enhanced adhesion with the core wire at hightemperature, and includes, for example, a functional group which isgenerally known to be unstable at high temperature such as —COOM, —SO₃M,—OSO₃M, —SO₂F, —SO₂Cl, —COF, —CH₂OH, —CONH₂ and —CF═CF₂ and the like.Here, M is an alkyl group, a hydrogen atom, a metallic cation or aquaternary ammonium cation.

The number of groups at the above adhesion terminus in the FEP pellet ofthe present invention depends on the total content of the above adhesiontermini in the copolymer the kind of adhesion terminus, MFR, die swell,monomer composition and the like. In the case of, for example, —COF,—COOH and —CH₂OH, under conditions such that the total number of thesethree groups is 15 to 150 and the total number of —COF and —COOH groupis 2 to 25, preferably —COF is 0 to 5, —COOH is 2 to 25, and —CH₂OH is 0to 300. These are numbers of functional groups per 1,000,000 carbonatoms.

It has generally been believed that such adhesion terminus is preferablyand completely removed by carrying out a terminus stabilizing treatmentdescribed below in order to lower the volatile content. The FEP pelletof the present invention, which has a volatile content within the rangespecified herein, can have the above adhesion termini by controlling theterminus stabilizing treatment. According to the present invention,since the above-mentioned adhesion terminus provides sufficient adhesivestrength with the core wire, it is unnecessary to take special measuressuch as annealing in order to obtain the desired adhesion strength withthe core wire.

(ii) An average number of cone-breaks in the insulating material is oneor less per 50,000 ft of the coated core wire.

As used herein, a cone-break is an irregularity of the surface in whichthe insulating material does not completely cover the core wire.

Provided that the number of cone-breaks is within the above-mentionedrange, complete covering can be carried out even if a coating speed ashigh as 2,800 ft/min or more is employed, so that a high-qualityinsulated cable is obtained. Preferably, the number of cone-breaks isone or less on average per 100,000 ft of the coated core wire, morepreferably one or less on average per 150,000 ft of the coated corewire.

The FEP pellet of the present invention has a volatile content of 0.2%by weight or less. Generally, the volatile components originate from theabove adhesion terminus, decomposed portions of the polymer main chain,an oligomer generated during polymerization, a residual solvent due toinsufficient deaeration during melt-pelletization and the like. If thevolatile content exceeds 0.2% by weight, foam generates in the aboveinsulating material, and together with the occurrence of the thinning ofthe insulating material due to draw-down, spark-out and thus cone-breaksoccur. Thus, molding is unstable, and it becomes difficult to coat athigh speed. The volatile content may be, for example, 0.07% by weight ormore as long as it is within the above-mentioned range, and preferably0.07 to 0.2% by weight. If the volatile content is less than the lowerlimit thereof, there are cases in which the above-mentioned adhesiontermini are too few. Hence, the adhesion between the above insulatingmaterial and the core wire is lessened to result in molding defects andeven the moldability is not sufficiently improved in some cases.

Preferably, the FEP pellet of the present invention has an MFR of 30(g/10 min.) or more. When the MFR is within the above-mentioned range,sufficient draw-down is possible. Thus, it becomes possible to maintainhigh productivity. Further, even if slight melt fracture occurs duringextrusion, self-flow tends to occur by the time it cools down andsolidifies, whereby the insulating material thus obtained has a smoothsurface with no melt fracture marks. It thus becomes easy to carry outcoating at a high-speed as mentioned above. If the MFR is less than 30(g/10 min.), the extent of melt fracture becomes severe, cone-breaks orspark-out due to melt fracture may be observed in some cases, and ittends to be difficult to increase the coating speed. The MFR may be 50(g/10 min.) or less within the above-mentioned range, and morepreferably is 30 to 45 (g/10 min.) since it becomes easy to increase theabove coating speed.

Preferably, the FEP pellet of the present invention has a die swellvalue of 18% or more. The die swell is generally an index for themolecular weight distribution, and the greater the die swell, the widerthe molecular weight distribution. On the contrary, the smaller the dieswell the narrower the molecular weight distribution. However, strictlyspeaking, the die swell reflects the amount of components that exhibitstrain hardening. Strain hardening reflects a force that resistselongation melt breakage. Namely, the die swell is an index showingresistance to elongation melt breakage.

Therefore, if the die swell is within the above-mentioned range in thepresent invention, the component that exhibits the above-mentionedstrain hardening is present in large amount, so that it is difficult formelt fracture to occur under a flowing status during extension such asdraw-down. The die swell value is more preferably 18 to 35%. If it isless than 18%, melt-tension during molding becomes small so thatcone-breaks easily occur, and if it exceeds 35%, the melt-tension is toohigh, so that irregularities occur on the insulating material thusobtained to thereby amplify variations in the wire diameter. Hence, themoldability tends to be unstable. More preferably, the die swell valueis 20 to 28%.

A process for controlling the die swell of the FEP pellet of the presentinvention within the above-mentioned range is not specifically limited.For example, a known method for increasing strain hardening can be used,and an example thereof includes addition of ultra-high molecular weightmaterial, addition of long-chain branched high molecules or the like.

As an index of the molecular weight distribution, the ratio of weightaverage molecular weight/number average molecular weight (Mw/Mn) isgenerally known. This index has often been considered as showingresistance to melt fracture. However, in principle, this understandingis not exactly correct. On the other hand, die swell is useful as anindex showing resistance to melt fracture as mentioned above.Accordingly, even if the Mw/Mn value is as small as, for example, lessthan 2, there is a difference in the die swell value, and hence adifference in frequency of occurrence of melt fracture in some cases.For the FEP pellet of the present invention, if the die swell ispreferably 18% or more, the Mw/Mn value is not limited.

The die swell is also useful since it can be measured easily asdescribed below. In contrast, the Mw/Mn value can be obtained, forexample, by using the method of W. H. Tuminello which comprises assuminga certain statistical distribution of the viscoelastic behavior whichmeans a certain molecular weight range (Plym. Eng. Sci. 26, 1339(1986)). However, this method is not sufficient as a scale for strainhardening.

Preferably, the FEP pellet of the present invention has a MIT bendinglife of 4,000 cycles or more. If the MIT bending life is within theabove range, the insulating material thus obtained maintains superiorresistance to stress cracking and resistance to brittleness, so thatexcellent toughness can be obtained even if it is coated relativelythick. If this value is less than 4000 cycles, problems may arise whenthe cable is used at normal temperature or higher. If the MIT bendinglife is within the above range, it may be 7,000 cycles or less since ithas sufficient resistance to stress cracking. The MIT bending life ispreferably 4,200 to 7,000 cycles, more preferably 4,400 to 6,500 cycles.

The method for controlling the MIT bending life of the FET pellet of thepresent invention within the above-mentioned range is not particularlylimited. For example, a conventionally well-known method for impartingresistance to stress cracking can be used.

The method for manufacturing the FEP pellet of the present invention isnot particularly limited and, for example, a conventionally well-knownmethod can be used. Such method may comprise melting and kneading aresin powder obtained in an extruder or the like while heating, forexample, at a temperature not lower than the melting temperature of theFEP as well as at a temperature lower than the decomposition temperatureof the above FEP, to thereby pelletize the resin powder.

The method for polymerizing the above FEP is not particularly limited,and an ordinary method of polymerizing monomer components such asemulsion polymerization, suspension polymerization, solutionpolymerization, block polymerization, gas phase polymerization or thelike can be used.

An example of a chain transfer agent that can be used to polymerize theabove-mentioned FEP includes a liquid chain transfer agent such asisoparaffin, carbon tetrachloride, diethyl malonate, mercaptan, diethylether, alcohol and the like. Further, as a gaseous chain transfer agent,methane, ethane and the like can be used.

Ammonium persulfate and/or potassium persulfate can be used as apolymerization initiating agent to start the polymerization reaction byinitial charging. Although the initially charged polymerizationinitiating agent is consumed or decomposed with progression of reactionfrom just after the start of the reaction, by adding an appropriateamount continuously, the molecular weight, molecular weight distributionand the like can be controlled.

If suspension polymerization is used, diacyl peroxide is preferably usedas the polymerization initiating agent. A suitable polymerizationinitiating agent is fluorine-containing peroxide represented by theformula:

(R_(f)COO)₂

where R_(f) is a perfluoroalkyl group, a hydrofluoroalkyl group or aperchlorofluoroalkyl group.

In the above-mentioned production process, the FEP pellet can beadjusted so as to obtain a volatile content, adhesive strength, MFR, dieswell, MIT bending life and the like each within the above-mentionedranges, for example, by using the processes described below, as needed.

As mentioned above, for enabling the FEP pellets to have an adhesivestrength within the above-mentioned range, the FEP has theabove-mentioned adhesion factors. Since the above adhesion factors areunstable with heating, volatile content can be used as one of indexes toexpress the content of the adhesion factors. On the other hand, in thepresent invention, in order to prevent spark-out and cone-breaks, thevolatile content has an upper limit. Thus, the above FEP pellet containsa suitable amount of the above adhesion factor as well as a volatilecontent falling within the above range. The resulting FEP pellet issuperior in terms of both adhesive strength and reduced cone-breaks.

In order to obtain an FEP of the invention having the above adhesionfactor, a conventional terminus stabilizing treatment, which is carriedout during the polymerization process to stabilize the adhesion factors,may be omitted if needed. For example, this process is suitable if theabove FEP pellets having a volatile content within the above range canbe obtained without carrying out the above terminus stabilizingtreatment. An example of the above-mentioned terminus stabilizingtreatment includes a process which comprises adding a chain transferagent during polymerization, a process which comprises carrying out heattreatment after emulsion polymerization, a process which comprisescarrying out high-temperature heat treatment after polymerization or thelike.

In some cases, the above-mentioned terminus stabilizing treatment may becarried out so that not all the above-mentioned adhesion termini arestabilized but a suitable number remain to thereby decrease the extentof foaming. This is achieved by reducing the addition amount of a chaintransfer agent, or by reducing the time or temperature of the terminusstabilizing treatment. This is suitable, for example, in a case wherethe volatile content may exceed the upper limit of the above rangeunless a terminus stabilizing treatment (with reduced extent) is carriedout.

The FEP pellet of the present invention can be adjusted to have amelting point or MFR within the above-mentioned range by adjusting theaddition amount of the chain transfer agent or the polymerizationinitiating agent during the polymerization reaction (to thereby adjustthe molecular weight and molecular weight distribution of the abovefluorine-containing copolymer). The melting point or MFR can also be setby adjusting the monomer composition of the above-mentioned FEP.

For the FEP pellet of the present invention, when a resin powderobtained by polymerization is pelletized, it is possible to adjust theextent of reduction of the volatile content index and increase in MFR byadjusting the heating temperature, heating time and the like.

The method of coating extrusion using the FEP pellet of the presentinvention is not particularly limited, and, for example, aconventionally well-known method can be used. A conventional extruderfor coating cables may be used without particular modification. Thecoating speed is not particularly limited, but at a high speed such as2,500 to 3,500 ft/min and in particular, 2,800 to 3,000 ft/min, theadvantageous effects of the FEP pellet of the present invention, namely,excellent adhesive strength with the core wire and no occurrence ofcone-breaks, can be realized. Needless to say, the FEP pellet of thepresent invention can also be effectively used in coating extrusion atlower coating speeds such as less than 2,500 ft/min. which have beenemployed in conventional coating extrusion, for example, approximately1,000 to 2,500 ft/min.

The insulated cable obtained by the above-mentioned coating extrusion isnot particularly limited as long as it comprises a core wire and aninsulating material to cover the same, and includes, for example, acable, a wire and the like. Examples thereof include an insulatedcommunication wire and, for example, a cable connecting a computer andperipherals thereof, including a data transfer cable such as a LAN cablecan be mentioned. It is also suitable as a plenum cable.

In the present specification, the adhesive strength between the aboveinsulating material and the core wire, volatile content, MFR, die swelland MIT bending life are defined and measured as follows.

A) Bonding Strength, Cone-breaks and Spark-out

A-1) Conditions of Coating Extrusion of the Cable for Measurement

The above insulating materials obtained by molding under the followingconditions are evaluated.

Core wire standard: AWG24

Core wire diameter: 20.1 mil (=approximately 0.51 mm)

Thickness of the insulating material: 7.5 mil (approximately 0.19 mm)

Diameter of the insulated cable: 35.1 mil (approximately 0.89 mm)

Conditions of coating extrusion

Cylinder diameter: 50 mm

L/D: 30

Temperature (° C.):

Z1: 338 Z2: 360 Z3: 371 Z4: 382 Z5: 399

CL: 404 AD: 404 HD: 404 DIE: 404

Preheating of the core wire: 165° C.

Core length: 4.7 cm

Length of air cooling: 20 ft

Inner diameter of the die: 8.71 mm, Outer diameter of the chip: 4.75 mm

A-2) Cone-breaks of the Insulating Material

Measurement is made using a KW32 TRIO instrument made by Zumbach.

A-3) Spark-out of the Above-mentioned Insulating Material

Measurement is made using a Model HF-20-H (2.0 KV) instrument made byCLINTON INSTRUMENT COMPANY.

A-4) Adhesive Strength of the Above Insulating Material with the CoreWire

Test specimens are prepared by cutting the insulated cable obtained inthe above manner to a length of 15 cm and peeling the insulation by 7.5cm from one end with 7.5 cm remaining, such that the specimens comprisea 7.5 cm-long insulated portion and a 7.5 cm-long uncovered core wireportion.

For the measuring device, a tensile tester type 4301, product ofInstron, and a metal plate having a columnar hole as shown in the Figureare used.

Figure: Metal plate having a columnar hole

Length: 5 cm

Width: 1 cm

Thickness: 0.20 cm

Inner diameter of the columnar hole: 0.0530 cm

The illustrated metal plate is fixed to a fixed chuck (lower chuck inthis case) of the tensile tester so that the columnar hole is parallelto the tensile direction. The test specimens thus prepared are eachinserted into the hole of the fixed metal plate from its lower endtoward the upper end so that the peeled core wire portion protrudes fromthe top end of the hole of the metal plate with the insulated portionlocated below the hole. This is because the diameter of the insulatedportion is larger than that of the hole. The uncovered core wire, whichprotrudes from the top of the hole, is clamped with a movable chuck(upper chuck in this case) and is moved upward at the speed of 50mm/min. Since the core wire is smaller than the columnar hole, only thecore wire is pulled up while peeling off the above insulating material.The force measured during this time is read. The tests are carried outfor five specimens, and the average value is taken for three specimensexcluding the two specimens having maximum and minimum values.

B) Volatile Content

Using an electric oven having a turntable, the volatile content (% byweight) is measured as follows. That is, as a sample, 20±0.1 g of theabove FEP in the form of pellet is weighed out using a precision balancecapable of measuring to an accuracy of 0.1 mg and in an aluminum cupthat has been baked at 370° C. beforehand in an empty state. The letterA used herein indicates the weight of the baked aluminum cup, and theletter B indicates the weight of the aluminum cup in which the sample isplaced. As for B, two aluminum cups containing a sample per each sampleto be measured are prepared and an average value of two cups is used. Onthis occasion, a standard sample having a known volatile content is alsoweighed out at the same time as a control. These are quickly put on theturntable of the electric oven, which has been heated to 370° C. At thistime, the rotation speed of the turntable is 6 rpm.

30 minutes after the internal temperature has returned to 370° C., thecups containing simple product removed from are taken out of the ovenand quickly put in a desiccator, in which they are left for over an hourto cool down. Then, the weight of the sample is accurately weighed bythe above precision scale. The letter C denotes their weights at thistime.

The weight reduction of the sample after heating for 30 minutes at 370°C. is calculated by the following formula and is referred as thevolatile content (% by weight):

Volatile content (% by weight)=((B−C)/(B−A)×100

C) MFR

Using a melt index tester in compliance with ASTM D 1238-98 or JIS K7210, approximately 6 g of resin is charged into a cylinder having aninner diameter of 0.376 inch and maintained at 372° C.±0.5° C., andallowed to adjust for five minutes until an equilibrium temperature isreached. Then, the resin is extruded through an orifice having adiameter of 0.0825 inch and a length of 0.315 inch under a piston loadof 5,000 g, and the weight of the resin (unit: g) collected in a unitperiod of time (usually 10 to 60 seconds) is measured. The threemeasured values are averaged and converted to the extrusion amount per10 minutes as the MFR (unit: g/10 min.).

D) Die Swell

Die swell is defined as “% expansion” in Japanese Kokoku PublicationSho-48-20788, and it is expressed as “expansion rate” in Japanese KokokuPublication Hei-2-7963. The die swell is measured as follows.

Using the same melt index tester as used above in the measurement ofMFR, approximately 6 g of resin is charged into a 0.376-inch cylindermaintained at 372° C.±0.5° C., and is allowed to adjust for five minutesuntil an equilibrium temperature state is reached. Then, the resin isextruded through an orifice for die swell measurement having a diameterof 1 mm (error +0.002 mm or less) and a land length of 1.05 mm (error±0.05 mm) under a piston load of 5,000 g. After the extruded strand iscooled to room temperature, its diameter is measured.

The strand length in this case is 30±5 mm, and the diameter at a portionapproximately 5±1 mm above the front end (the portion that has beenextruded first) is considered to be the diameter of the strand. Thediameters of three such strands collected at the same time are measuredand averaged, and the die swell is calculated according to the followingequation.

Die swell (%)=((SD−OD)/OD)×100

SD: diameter of the strand (average of three strands)

OD: diameter of the orifice (1.000±0.002 mm)

E) MIT Bending Life

This measurement is made using a standard MIT folding endurance testerdescribed in ASTM D 2176-97. The measurement is made using acompression-molded film quickly cooled in cold water. The film has athickness of approximately 0.008±0.0005 inch (0.20±0.013 mm).

F) Number of Groups at the Adhesion Terminus

The resin is compression-molded at as low a temperature as possiblewithin a range such that the resin is moldable between the melting pointof the resin and 350° C. to thereby form a film having a thickness of250 to 300 μm. The infrared absorption spectrum of this film is measuredand compared with the infrared spectrum of a sample not containing endgroups that are present in this film to determine the kind of endgroups. Then, from their differential spectra, the number of adhesiontermini is calculated according to the following equation.

Number of end groups (per 10⁶ carbon atoms)=(l·k/t)

l: absorbance

k: corrective coefficient

t: film thickness (mm)

The corrective coefficients of the target end groups are shown below.These corrective coefficients are determined based on the infraredabsorption spectrum of model compounds for calculating the number of endgroups per 10⁶ carbon atoms. The infrared absorption spectrum aremeasured by scanning 10 times using a Perkin-Elmer (FT-IR Spectrometer1760X).

Number of absorbed Terminus group waves (cm⁻¹) Corrective coefficient—COF 1883 388 —COOH (free) 1815 440 —COOH (bonded) 1775 440 —CH₂OH 36482300

EFFECT OF THE INVENTION

The FEP pellet of the present invention restricts the volatile contentwithin the above-specified range, increases the adhesive strengthbetween the above insulating material and the core wire to within theabove-specified range, and suppresses the occurrence of cone-breakswithin the above-specified range. Thus, even in cases where the coatingextrusion is carried out at a high coating speed as described above,melt fracture will not occur and high-speed coating extrusion can becarried out. Further, the above insulating material and the insulatedcable thus obtained have superior physical properties such as mechanicalstrength and electrical properties, so that they can sufficientlywithstand long-term use.

Thus, the FEP pellet of the present invention can be suitably used forcoating extrusion of cables. The effects of the present invention aremanifest especially in case of high-speed molding.

The insulating material formed from the above FEP pellet is also anembodiment of the present invention.

An insulated cable formed from the above FEP pellet is an additionalembodiment of the present invention.

EXAMPLES

The present invention is described in further detail by means of thefollowing Examples. However, the present invention should not beconstrued as being limited thereto.

Example 1

(Manufacturing of Modified FEP Powder)

310 kg of pure water and 200 g of ω-hydrofluorocarboxylic acid werecharged into a 1,000 L glass lined autoclave having a stirrer. Afterpurging with nitrogen, a vacuum was established, and 320 kg ofhexafluoropropylene (HFP) and 3 kg of perfluoro(propyl vinyl ether)(PPVE) as a perfluoro(alkyl vinyl ether) (PFAVE) were charged thereto.Stirring was started, the temperature of the polymerizing vessel was setat 35.0° C., and the pressure was increased to 10.5 MPaG withperfluoroethylene (TFE). Then, when 3.8 kg ofdi-(ω-hydrodecafluoroheptanol)peroxide (DHP) diluted to approximately 8wt % with perfluorohexane was added, the reaction began immediately.During the reaction, TFE was additionally charged in an amount of 310 kgto maintain the pressure in the autoclave at 10.5 MPaG. During thereaction, when the amount of TFE supplied reached 25 wt %, 50 wt % and75 wt % of the reaction mixture, PPVE was charged in an amount of 800 gat each time interval. Also, 2 and 4 hours after the start of thereaction, DHP was added each time in an amount of 3.8 kg, and six hoursafter the start of the reaction and every two hours thereafter, DHP wasadded in an amount of 1.9 kg, respectively. In order to adjust themolecular weight, 1.3 kg of methanol was added as a chain transfer agent15 hours after the start of the reaction.

After a total reaction time of 28 hours, non-reacted TFE and HFP werereleased to obtain a granular powder. Pure water was added to thispowder, and after stirring and washing, it was removed from theautoclave. The powder was then dried for 24 hours at 150° C. to obtain385 kg of the above-mentioned modified FEP powder.

(Evaluation of the Physical Properties of the Modified FEP Powder)

The resulting modified FEP powder measured as described above had acomposition weight ratio of TFE:HFP:PPVE of 86.1:13.0:0.9, an MFR of33.7 (g/10 min.), a die swell of 32.3%, and a volatile content of 0.911%by weight.

(Pelletization and Evaluation of Physical Properties)

Using a 50 mm-diameter extruder having a barrel to die temperature setat 360° C., the above modified FEP powder was pelletized at a speed of12 kg/hour, and then deaeration was carried out for 8 hours at 230° C.For the pellets by the above-mentioned process, the MFR was 40.4 (g/10min.), the die swell was 22.1%, and the volatile content was 0.135% byweight. As the above adhesion termini, the numbers of the respectivegroups —COF, —COOH and —CH₂OH were measured as described above, and theresults are shown in Table 1.

(Coating Extrusion)

Thereafter, cable coating (molding) was carried out at respectivecoating speeds of 1,600, 2,000, 2400 and 2,800 ft/min. under theabove-described conditions. For each coating speed, the number ofspark-outs and cone-breaks as well as the adhesive strength between theabove insulating material and the core wire per 30 minutes of thecoating operation were evaluated as described above. The number ofspark-outs and cone-breaks convert to the number of spark-outs andcone-breaks per 100,000 ft of the coated cable. The results are shown inTable 1.

Example 2

Except that methanol was added in an amount of 1 kg, and the time atwhich methanol was added was changed to 16 hours after the start of thereaction, the procedure of Example 2 was the same as Example 1.

Example 3

Except that perfluoro(ethyl vinyl ether) (PEVE) was used instead ofPPVE, methanol was added in an amount of 1.1 kg, and the time at whichmethanol was added was changed to 16 hours after the start of thereaction, the procedure of Example 3 was the same as Example 1.

Example 4

Except that methanol was added in an amount of 2 kg, and the time atwhich methanol was added was changed to 20 hours after the start of thereaction, the procedure of Example was the same as Example 1.

Example 5

Except that PFAVE was not added, the procedure of Example 5 was the sameas Example 1.

Example 6

Except that the temperature of the polymerizing vessel was changed to34° C., PPVE was initially added in an amount of 4 kg and in amounts of1,000 g at the predetermined intervals after the start of the reaction,methanol was added in an amount of 8.1 kg, and the time at whichmethanol was added was changed to 10 hours after the start of thereaction, the procedure of Example 6 was the same as Example 1.

Comparative Example 1

Except that the extrusion speed for pelletization was changed to 17kg/hour, the procedure of Comparative Example 1 was the same as Example1.

Comparative Example 2

Except that methanol was added in an amount of 3.4 kg, and the time atwhich methanol was added was changed to 8 hours after the start of thereaction, the procedure of Comparative Example 2 was the same as Example6.

Comparative Example 3

Except that methanol was added in an amount of 0.8 kg, and the time atwhich methanol was added was changed to 16 hours after the start of thereaction, the procedure of Comparative Example 3 was the same as Example3.

Comparative Example 4

Except that the temperature of the polymerizing bath was changed to 34°C., methanol was added in an amount of 5.9 kg, and the time at whichmethanol was added was changed to 8 hours after the start of thereaction, the procedure of Comparative Example 4 was the same as Example1.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 1 2 3 4 PolymerizationWater (kg) 310 310 310 310 310 310 310 310 310 310 Temperature (° C.) 3535 35 35 35 34 35 34 35 34 HFP (kg) 320 320 320 320 320 320 320 320 320320 Pressure (MPaG) 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5 10.5PPVE (kg) 3 3 — 3 0 4 3 4 — 3 PPVE (each (g) 800 800 — 800 0 1000 8001000 — 800 interval) PEVE (initial (kg) — — 3 — — — — — 3 — period) PPVE(each (g) — — 800 — — — — — 800 — interval) DHP (initial (kg) 3.8 3.83.8 3.8 3.8 3.8 3.8 3.8 3.8 3.8 period, after 2 & 4 hours) DHP (after 6(kg) 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 hours) Methanol (kg) 1.3 11.1 2 1.3 8.1 1.3 3.4 0.8 5.9 Time period after (hours) 15 16 16 20 1510 15 8 16 8 reaction when methanol is added Total period of (hours) 2828 27 28 30 25 28 24 25 22 reaction Yield (kg) 358 359 351 362 350 357358 358 354 358 Powder MFR (g/10 33.7 26 25 30 28.5 27.7 33.7 27.6 22.618.5 min.) HFP/PPVE(or (% by 13.0/0.9 13.0/0.9 13.0/0.9 13.0/0.913.7/0.0 11.5/1.2 13.0/0.9 11.5/1.2 13.0/0.9 11.7/1.0 PEVE) weight) Dieswell (%) 32.3 34.2 33.8 48 36.2 25.2 32.3 24.6 31.3 32.1 Volatilecontent (% by 0.911 0.925 0.835 0.925 0.865 0.655 0.911 0.672 0.8350.593 weight) Pelletization Extrusion (° C.) 360 360 360 360 360 360 360360 360 360 temperature Extrusion speed (kg/hour) 12 12 12 12 12 12 1712 12 12 Deaeration time (hours) 8 8 8 8 8 8 8 8 8 8 Pellet MFR (g/1040.4 32.5 31.1 37.5 35.1 35.6 36.1 33.2 28.3 25 min.) Die swell (%) 22.123.5 22.6 34 25.6 22.3 24.1 16 20.8 24.7 Volatile content (% by 0.1350.162 0.152 0.144 0.142 0.147 0.213 0.128 0.158 0.162 weight) Meltingpoint (° C.) 247.2 248.1 246.5 247.5 250.1 255.3 247 256.1 248.1 256.2Adhesion terminus group (per 10⁶ carbon atoms) —COF (number) 4 3 4 4 3 41 1 3 2 —COOH (number) 3 2 1 2 6 3 3 2 1 1 —CH₂OH (number) 65 53 67 5349 125 72 112 46 105 MIT bending life (cycle) 4808 6142 4569 5316 11135515 4725 5146 4569 6497 Evaluation of insulated cable for each coatingspeed (ft/min) (number per 100,000 ft for spark-out and cone-breaks)1600 Spark- (number) 1 2 1 1 1 0 12 0 1 8.6 (ft/ outs min.) Adhesive(kg) 2.2 — — — — — — — — — strength Cone- (number) 0 0 0 0 0 0 0 0 0 0breaks 2000 Spark- (number) 2 3 1 1 6 4 15 4.7 1 11.8 (ft/ outs min.)Cone- (number) 0 0 0 0 0 0 1 0 0 0 breaks 2400 Spark- (number) 4 3 2 3 73 — 8.7 2 17.5 (ft/) outs min.) Adhesive (kg) 1.9 — — — — — — — — —strength Cone- (number) 0 0 0 0 0 0 — 0 0 1 breaks 2800 Spark- (number)6 4 3 4 6 4 — 10 14 25 (ft/) outs min.) Adhesive (kg) 1.7 1.5 1.4 1.81.5 1.3 — 1.1 1 0.8 strength Cone- (number) 0 0 0 0 0 0 — 3 4 7 breaks

In Table 1, HFP/PPVE (PEVE) is the content of HFP and PPVE or PEVE units(% by weight) in terms of all monomer components constituting the FEPpellets thus prepared.

Table 1 shows that all of the Examples of the present invention aresuperior, in terms of spark-out, cone-breaks and adhesive strengthbetween the insulating material and the core wire, whereas ComparativeExample 1 having a high volatile content, Comparative Example 2 having alow die swell, and Comparative Examples 3 and 4 having a large MFR wereeach inferior in at least one of the evaluation criteria. Also, it isapparent that Example 3, which has a large MFR as compared toComparative Example 3, was free of cone-breaks.

Example 7 and Comparative Example 5

(Manufacturing of a Dispersion Solution of an FEP Copolymer ContainingFEP Copolymer Seed Particles)

A 50-L stainless autoclave with a stirrer was deaerated, and thencharged with 30 kg of deaerated distilled water and 8 kg of a 10% byweight aqueous solution of fluorine surfactant (C₇F₁₅COONH₄). Further, 5kg of HFP (liquid) and then a gaseous TFE-HFP mixed monomer(TFE:HFP=86:14 in weight ratio) were charged to the autoclave. Thetemperature was raised gradually with stirring. As a result, thepressure of the atmosphere in the autoclave was raised to 1.5 MPaG at95° C.

As an initiator, an ammonium persulfate aqueous solution (APS)(10% byweight) was charged in an amount of 3.5 kg to start the reaction. Theabove mixed monomer was continuously supplied to keep the pressure at1.5 MPaG. Stirring was stopped 30 minutes later, and non-reacted TFE andHFP were collected to obtain 31.4 kg of a dispersion solution of an FEPcopolymer having a polymer solids content of 4.5% by weight. Thisdispersion solution is referred to as an FEP copolymer dispersionsolution containing FEP copolymer seed particles.

Some of the dispersion solution was coagulated using nitric acid toobtain a white powder. The composition of the copolymer thus obtainedhad a weight ratio of TFE:HFP=86.0:14.0. It was impossible to measurethe MFR, die swell and volatile content.

(Manufacturing of Modified FEP Powder)

Next, a 50-L stainless autoclave equipped with a stirrer was deaeratedand charged with 30 kg of deaerated distilled water and then with 1 kgof the FEP copolymer dispersion solution containing FEP copolymer seedparticles that had been prepared beforehand. Further, 6.9 kg of HFP(liquid), and then 0.2 kg of PPVE, and then a gaseous TFE-HFP mixedmonomer (TFE:HFP=86:14 in weight ratio) were charged to the autoclave.The temperature was raised gradually with stirring, and the pressure ofthe atmosphere in the autoclave was raised to 4.2 MPaG at 95° C.

APS (10% by weight; 1.0 kg) was then charged to the autoclave and thereaction was started. Upon start of the reaction, a gaseous TFE-HFPmixed monomer having the same composition as mentioned above wascontinuously supplied so as to maintain a pressure of 4.2 MpaG. Further,each time the amount of the TFE-HFP mixed monomer that has been suppliedduring the reaction reached 25, 50 and 75 wt % of the reaction mixture,PPVE was charged in an amount of 20 g at each time interval.Polymerization was continued until the solids content of the polymerreached 20% by weight. The total reaction time was 45 minutes.

Thereafter, non-reacted TFE and HFP were collected, and the dispersionwas removed and coagulated with nitric acid to obtain a white powder.The amount of the modified FEP after drying was 7.7 kg.

(Evaluation of the Physical Properties of the Modified FEP Powder, EndStabilizing Treatment, and Pelletization)

For the modified FEP obtained by the above methods, the weight ratio ofTFE:HFP:PPVE was 86.1:12.9:1.0, MFR was 33 (g/10 min.), and the dieswell was 34.0%.

As an end stabilizing step, this modified FEP powder was subjected towet heat treatment while varying the treatment period, and then melted,pelletized and deaerated to obtain two samples. The sample which wassubjected to wet heat treatment for 45 minutes at 380° C. underatmospheric pressure is referred to as Example 7, and the sample whichwas subjected to wet heat treatment for an hour is referred to asComparative Example 5. The results are shown in Table 2.

(Evaluation of Physical Properties and Coating Extrusion)

In the same manner as in Example 1, various physical properties weremeasured, cable extrusion coating was carried out, and various criteriawere evaluated including spark-out. The results are shown in Table 2.

TABLE 2 Comparative Example Example 7 5 Polymerization Water (kg) 30 30Seeds (4.5%) (kg) 1 1 Temperature (° C.) 95 95 HFP (kg) 6.9 6.9 Pressure(MPaG) 4.2 4.2 PPVE (initial period) (kg) 0.2 0.2 PPVE (each interval)(g) 20 20 APS (10%) (kg) 1 1 Total reaction time (hour) 45 45 Yield (kg)7.7 7.7 Powder MFR (g/10 min.) 33 33 HFP/PPVE (% by weight) 12.9/1.012.9/1.0 Die swell (%) 34 34 Volatile content (% by weight) — —Pelletization Wet heat treatment time (min.) 45 60 Extrusion temperature(° C.) 360 360 Extrusion time (kg/hour) 12 12 Deaeration time (hour) 8 8Pellet MFR (g/10 min.) 34.5 35.1 Die swell (%) 24 21 Volatile content (%by weight) 0.13 0.06 Melting point (° C.) 248.5 247.6 Adhesion terminusgroups (per 10⁶ carbon atoms) —COF (number) 0 0 —COOH (number) 21 0—CH₂OH (number) — — MIT bending life (cycles) 6127 6525 Evaluation ofcovered cable for each covering speed (ft/min) 1600 (ft/min) Spark-outs(number) 3 2 Adhesive strength (kg) — 1.9 Cone-breaks (number) 0 0 2000(ft/min) Spark-outs (number) 9 4 cone-breaks (number) 0 0 2400 (ft/min)Spark-outs (number) 10 2 Adhesive strength (kg) — 1 cone-breaks (number)0 0 2800 (ft/min) Spark-outs (number) 15 5 Adhesive strength (kg) 1.10.5 cone-breaks (number) 0 0 Note: Spark-outs and cone-breaks arenumbers per 100,000 ft.

In Table 2, HFP/PPVE is the content of HFP and PPVE units (% by weight)of the above modified FEP thus obtained in terms of all constituentmonomer components. Table 2 shows that Comparative Example 5 in whichthe end stabilizing treatment time was long had almost no adhesivestrength and was inferior in adhesion strength to Example 7 in which theend stabilizing treatment time was short.

It should further be apparent to those skilled in the art that variouschanges in form in detail of the invention as shown and described abovemay be made. It is intended that such changes be included within thespirit and scope of the claims appended hereto.

What is claimed is:
 1. An FEP pellet having a volatile content of 0.2% by weight or less, wherein said FEP pellet satisfies the following requirements (i) and (ii) when used to form an insulating material coating a core wire by extrusion coating at a coating speed of 2,800 ft/min.: (i) an adhesive strength between said insulating material and said core wire of 0.8 kg or more; and (ii) an average number of cone-breaks in said insulating material of one or less per 50,000 ft of the coated core wire.
 2. The FEP pellet as claimed in claim 1, having a melt flow rate of 30 (g/10 minutes) or more.
 3. The FEP pellet as claimed in claim 1, having a die swell of 18 to 35%.
 4. The FEP pellet as claimed in claim 2, having a die swell of 18 to 35%.
 5. The FEP pellet as claimed in claim 3, having an MIT bending life of 4,000 cycles or more, and said FEP pellet comprising a tetrafluoroethylene/hexafluoro-propylene copolymer modified with perfluoro(alkyl vinyl ether).
 6. The FEP pellet as claimed in claim 4, having an MIT bending life of 4,000 cycles or more, and said FEP pellet comprising a tetrafluoroethylene/hexafluoro-propylene copolymer modified with a perfluoro(alkyl vinyl ether).
 7. The FEP pellet as claimed in claim 5, wherein said perfluoro(alkyl vinyl ether) is perfluoro(propyl vinyl ether).
 8. The FEP pellet as claimed in claim 6, wherein said perfluoro(alkyl vinyl ether) is perfluoro(propyl vinyl ether).
 9. An insulating material formed from the FEP pellet as claimed in claim
 1. 10. An insulated cable comprising a core wire extrusion coated with an insulating material prepared from the FEP pellet as claimed in claim
 1. 11. A process for insulating a core wire, which comprises extrusion coating the core wire with a molten insulating material comprising the FEP pellet as claimed in claim
 1. 